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    Core Java Programming language Theory part only

    What is object oriented programming language discuss the feature of object

    oriented programming in respect of java.

    Object oriented programming is those type of programming in which methods and

    instance variable allocate distinct memory in different location and at the time of executionmake a link between them to create a complete object, which is the basic component of the

    object oriented programming. Methods allocates there respective memory at the time of

    class declaration and instance variable allocates there memory at the time of object

    creation at execution time. After allocation of memory by instance variable a binding

    process is performed between methods and instance variable after that a complete object is

    created which is the basic component of the object oriented programming.

    Object are that type of element which can be uniquely identified using some identifying

    properties. Alternately we can say that Object is that type of element which can be

    distinguished from other element.

    It is also possible to say that class type variable is called object every object must have

    some identifying properties to uniquely identify a particular object. The identifying

    properties are called instance variable in java. So we can say that object are the

    encapsulated form of methods and instance variable. Every object must have some function

    to perform task. The functions present in object are called methods in java. Finally we can

    say that object are the encapsulated form of instance variable and there associated methods.

    The methods present in object are common for a set of object of same type. So, methods arecreated a single copy for a set of object, and they are linked with a set of instance variable.

    After creating object .instance variable are distinct for all the object and they are used to

    identify a particular object. In java four visibilities are present to provide to access right of

    instance variable and methods. The visibilities are as:-

    Visibility

    Private

    Default /Friendly

    Protected

    Public

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    Core Java Programming language Theory part only

    1. Private :Private visibilities specify that this section can not be access directly from the outside of the object but can be access from any where of object. That means, private section

    can be access from public, friendly and protected section of the object .So finally we can say

    that private section is visible in a single object at a time. Private section is not inheritable.That means function and variable present in the private section

    2. Default / Friendly :The default visibility is friendly in java. The default section can beaccess directly from the out side of the object. That means the default section can be access

    in entire program of a particular package (Collection of related class). Default section can

    be inherited into the subclass. That means variable and function present in the default

    section. Default section can be override and overwrite in the subclass default section can

    contains instance variable as well as methods.

    3. Protected :The protected section can also be access directly from the out side of theobject. That means the protected is visible in entire program in a particular package same

    as default. We know that the default section can not be access in another package but

    protected section can be access other package in subclass. protected section can not be

    access in non subclass in other package protected section can contains methods as well as

    instance variable protected can be inherit into sub class and the method and instance

    variable can be overwrite and override in the subclass . To protect overwriting and

    overriding the final keyboard are used before the visibility of function and variable.

    4. Public : The most visibility in java program is public. The public visibility specifies that thissection can be access from any where in java programming. Public section can directly

    access from out side of object. That means public section is visible in the entire program in

    a particular package. Public section can also be visible in subclass as well as in non

    subclass in other package. Public section can be inheriting subclass and can be modified as

    the requirement of the user.

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    Core Java Programming language Theory part only

    Feature of object oriented programming in respect of JAVA

    Encapsulation or Abstraction.

    Compiled and interpreted.A.P.I(Application program interface ).

    Simple secured and robust.

    Platform independence.

    Generic programming.

    Class and object.

    Multithreading.

    Polymorphism.Distributed.

    Inheritance.

    Constructor.

    Exception.

    Applet.

    Interface.

    Dynamic Package..

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    Core Java Programming language Theory part only

    The common name of a set of object is called class. It is also possible to say that class is the

    blue print or logical design and imaginary design of object. Alternately we can say that

    class is users define data type. Which is used to create new data type by encapsulating

    standard data type or other user define data type with there associated member function(methods) in a single unit as the requirement of the user

    .

    Class is an user define data type which is used to create new data type as the requirement ofthe user by encapsulating different standard data type or other user define data type in a

    single unit.

    It is also possible to say that common name of set of object is called class. alternately we can

    say that class is the blue print or logical design of object in the encapsulated form of

    instance variable and methods so in class methods and variable are present with there

    appropriate type . In java 4 types of visibility are present and they are applied in class indesign time. It is also known to us that the method are common for a set of object so all

    method should be public.

    1. Private : Private visibility specifies that this section can not be access directly from theout side of the object but can be access from any where of object. That means private section

    can be access from public, friendly and protected section of the object .So finally we can say

    that private section is visible in a single object at a time. Private section is not inheritable.

    That means function and variable present in the private section.

    2. Deffoult/Friendelly : The default visibility is friendly in java. The default section can beaccess directly from the out side of the object. That means the default section can be access

    in entire program of a particular package (Collection of related class). Default section can

    be inherited into the subclass. That means variable and function present in the default

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    Core Java Programming language Theory part only

    section. Default section can be override and overwrite in the subclass default section can

    contains instance variable as well as methods.

    3. Protected : The protected section can also be access directly from the out side of the

    object. That means the protected is visible in entire program in a particular package sameas default. We know that the default section can not be access in another package but

    protected section can be access other package in subclass. protected section can not be

    access in non subclass in other package protected section can contains methods as well as

    instance variable protected can be inherit into sub class and the method and instance

    variable can be overwrite and override in the subclass . To protect overwriting and

    overriding the final keyword are used before the visibility of function and variable.

    4. Public : The most visibility in java program is public. The public visibilities specify thatthis section can be access from any where in java programming. Public section can directly

    access from out side of object. That means public section is visible in the entire program ina particular package public section can also be visible in subclass as well as in non subclass

    in other package. Public section can be Inherit in subclass and can be modified as the

    requirement of the user. The common name of set of object is called class. It is also possible

    to say that class is the blue print or logical design and imaginary design of object.

    alternately be can say that class is an user define data type to create new data type by

    encapsulating standard data type or other user define data type with there associated

    member function (methods) in a single unit as there requirement of the user.

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    Core Java Programming language Theory part only

    Encapsulation is a binding process by which instance variable are bind with there associated

    method to create a complete object. We know that method are common for a set of object

    and they are created at the time of class declaration instance variable allocates there

    memory at the time of object creation dynamically at the time of execution after allocation of

    memory by the instance variable a link between instance variable and there associated

    method. This linking process is called Encapsulation or data binding. Which is perform

    dynamically at the time of execution this type of binding is called late binding or run time

    binding or execution time binding.

    The main purpose of encapsulation is to create a single unit by binding different instance

    variable and there function and extra cover is arise around the component of the object and

    concurrently the internal component are hidden from out side of the object. So we can say

    that encapsulation implemented data binding by providing appropriate access pacifiers to

    the component. Alternately we can say that encapsulation is used to implement data

    abstraction by hiding all components from out side of the object. Encapsulation creates a

    single unit and the unit can move independently from one location to another location.

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    Core Java Programming language Theory part only

    One of the most important features of any programming language is the reusability of code.

    Inheritance is process to implement the concept reusability of code. In java three techniques

    are used to implement the concept of reusability as:-

    1. Function.

    2. Inheritance.

    3. Package.

    Inheritance is a process by which we can create new classes using existing sub class is called

    super class and the new class is called base class. Alternately we can say that inheritance is

    a process to implement the concept of parent child relationship that means in java more

    than one super class can not be present for a single subclass. That means more than one

    super class can not inherit into a single subclass so multiple as well as hybrid inheritance

    can not be implemented in java. The main purpose of is to reuse the existing code present in

    super class in subclass the following advantage are present in inheritance.

    Usese

    1. Using interface existing code can be reuse in subclass so using the concept of

    inheritance programmers can develop large program very easily.

    2. Since code can be reuse in subclass productivity of programs is increase.

    3. If productivity is increase than the product cost will we decrease as well as

    maintenance cost also is decrease.

    4. If product cost and maintenance cost is minimum of a product than the

    acceptability of the product will be maximum.

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    Core Java Programming language Theory part only

    Type of inheritance

    Single or simple inheritance :Single or simple inheritance is those type of inheritance in

    which a single super class is used to create single subclass. Inheritance visibility ofcomponent can not be change in java graphically we can represent single/simple

    inheritance as.

    Multilevel inheritance :Multilevel inheritance is those type of inheritance in which a subclass is used as a super class to create another sub class .alternately we can say that

    multilevel inheritance is the representation of single inheritance. Graphically we can

    represent multilevel inheritance as.

    INHERETANCE

    SINGLE/SIMPLE

    MULTILEVEL

    HERERICAL/TREE STRUCTRE

    CLASS A

    CLASS B

    CLASS A

    CLASS B

    CLASS C

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    Core Java Programming language Theory part only

    Hierarchical/Tree structure inheritance : Hierarchical or tree structure inheritance isthose type of inheritance in which every subclass has only one super class. But every super

    class can be more than one sub class. graphically we can represent heretical/ tree structure

    inheritance as-

    In inheritance the concept of overwriting and overriding is arise if an instance variable is

    inherit from super class to subclass than in subclass new value can be supplied to the

    variable . the new overwrites the executing value of the variable .in the variable the concept

    of overwriting is implemented . If user wants to restrict the overwriting than the final

    keyword is used before the general declaration of the variable. The final variable can notcan not be overwrite in the subclass . That means final keyword create constant that values

    can not be changed in subclass. Function present in subclass can be inherited into subclass

    and new definition of the function can be created by the user in sub classes. the new

    definition of the function of subclass overwrite the executing definition of function in super

    class in subclass new definition is active and in super class the old definition is user want to

    protect function overwriting than the final keyword is used before the general definition of

    the function . The final keyword can not restrict the inheritance of the method and instance

    variable it only restrict only overwriting and overriding.

    If class is final than class can not be inherit into subclass the final keyword protect theextension of class in java subclass is the extension of the super class and so final keyword

    restrict the extension.

    A

    BC

    D E

    F G

    H

    I J K L M

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    Core Java Programming language Theory part only

    Abstract class : Abstract class is that type of the class which has no capacity to create object.Abstract class must be inherit into subclass to create existence of the abstract class. Abstract

    class is used as a super class in inheritance.

    Abstract Function : Abstract function is that type of function which has no definition orbody, if a class contains an abstract function than the class must be also an abstract.

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    Core Java Programming language Theory part only

    The general meaning of polymorphism is single interface multiple task. That means same thing

    different form. Poly means many and morph means form. That means polymorphism is those

    type of programming concept in which a particular function can take different form by

    taking different number of argument and different types of argument and can perform

    different task according to the form of the function. In java, polymorphism is only

    implemented using the concept of function overloading. There is no concept of operator

    overloading in JAVA. In real life different task are performed using a single function by

    supplying different arguments. to implement this concept in programming language the

    concept of function overloading is arise. The main purpose of polymorphism is to handle

    huge number of function.

    Function overloading is those type of programming system in which a particular function can

    take a different form by taking different number of arguments and different types of

    argument and can perform different task according to the form of the function. In JAVA

    function overloading is implemented using dynamic binding. Function overloading using

    early binding is not present using the concept of simple methods. Function overloading

    using early binding can be implemented in JAVA using the concept of static function.

    We know that functions are common for a set of object and they are allocate there respective

    memory at the time of class declaration. In JAVA objects are created at the time of execution

    usingnew operator. After that a binding process is performed between instance variables

    and there associated functions. This type of binding is calledlate bindingordynamic

    bindingorrun time bindingorexecution time binding.In JAVA generally methods are

    banded with instance variables using dynamic binding. So using simple methods, function

    overloading can be implemented using dynamic binding. Static function is called using the

    class name not the object of the class and can perform any operation on any type of instance

    variables using static function. Function overloading can also implement using early

    binding.

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    Core Java Programming language Theory part only

    JAVA programming language is platform independent. That means JAVA program can be

    executed on any operating system and on any computer with any internal architecture

    (configuration) provided the word length of the computer must be greater than or equal to16

    bit.Platform independent indicates that the program is not dependent on the operating

    system as well as internal architecture of the computers. JAVA program is not dependent on

    operating system of the computer as well as internal architecture of the computer system.

    But to execute the JAVA program the internal configuration is a matter, because the word

    length of the computer must be greater then or equal to 16 bit.Lessthen 16 bitcomputer isnot capable to execute the JAVA program. The main cause of platform independent of JAVA

    programming language is theJVM(Java Virtual Machine).

    JVMis anoperating system over the actual operating system.JVMis responsible to make

    compressibilityof the actual operating system with the JAVA program.JVMis interfacing

    between the actual operating system and the JAVA program. The internal coding system of

    JVMis Unicode, which is a 16 bitcoding system. So the interact number of all popular

    operating system is present inJVM, which can make a link with the physical operating

    system.

    JVMis divided in three parts

    (1) JIT (Just In time Compiler)

    (2) JAVA interpreter

    (3) JAVA Run Time Environment

    JAVA compiler is known as just in time compiler because all programs are just in time

    compiler because the entire program is not converted at a time to its equivalent object code

    (byte code). We know that compiler compile a program in to two phases. The first phase is

    known as analysis of source program. In this phase only syntax errors are checked. If syntaxerrors are present then appropriate error message is displayed. The second phase is called

    synthesis of object program. In this phase actual object code is created. But JAVA compiler,

    compile those segment which is to be executed in the next cycling.

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    JAVA compiler is not compile system defined function because they are in compiled form. That

    means JAVA compiler compile a particular segment at a time which is to be executed in the

    next cycling. JAVA compiler creates an intermediate code which is called byte code, written

    in Unicode system. The intermediate byte code is interpreted by the JAVA interpreter and

    creates appropriate object code understandable by the physical operating system. The JAVAcompiler is javacand the JAVA interpreter is java. JAVA interpreter interprets byte

    code to its equivalent object code and sent to the java run time environment and the java run

    time environment sent the interpreted object code to the operating system. Actually the

    operating system and the java interpreter is interfaced using java runtime environment. The

    main component present behind the platform independency is the java runtime environment.

    In this component all the kernel information of popular operating system are present at the

    time of installation. The kernel information is activated in the java runtime environment

    which is present in the internal operating system of the computer. The other kernels are

    temporarily deactivated.

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    Core Java Programming language Theory part only

    MULTITHREADINGMultithreading is those type of programming concept which is used to implement the concept of

    multitasking. Thread means, small program which can be executed at a time. Multithreading

    means more then one smalls program which are executed simultaneously not concurrently

    using small span of time and using the round robin scheduling. We know thatJVMis an

    operating system over the operating system, so in JVM a scheduler is present which is

    responsible to implement multithreading. In multithreading threads are differentstates :-

    1. New born state

    2. Ready state

    3. Running state

    4. Suspended state (temporarily)

    5. Terminated state or killed

    In JAVA a thread class is present which is used to implement the concept of multithreading. A

    run( ) function is present in the thread class which is used to run a thread. An init( )

    function is present, which is used to initialize a thread or new born thread is created.Start( ) function is also present, which is used to ready a thread to execute.Stop( ) function

    is present which is responsible to terminate a thread. For temporarily suspended of thread

    three functions are present and they are-

    1. Sleep

    2. Suspended

    3. Wait

    The sleep( ) is automatically deactivated after specific time given in millisecond in the sleep( )function. The suspended( ) function is deactivated when a resume( ) function is called. Wait

    function is deactivated when a notify( ) is called.

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    Core Java Programming language Theory part only

    Constructor is a special member function thats name is same with class name and that isautomatically invoked at the time of object creation. Construction has no return type even

    void. Construction can take argument or can not be. Constructor can not be declared in the

    private section of the class. But other visibility can be present with constructor. That means,

    constructor can be public, protected or friendly but can not be private. Constructor is

    divided in to two categories according to the argument. If the constructor can not take any

    argument then the constructor is called default constructor. If the constructor can take any

    argument then it is called parameterized constructor. If a class contains a parameterized

    constructor then the class should also contains a default constructor. There is no concept of

    copy constructor in JAVA. Constructor can not be inherit in the derived classes but can beinvoked from the sub-classes using the keywordsuper. If the super class contains a

    default constructor then it is not necessary to present a constructor in the sub-classes. If the

    super class contains a parameterized constructor, then in the sub class minimum one

    constructor must be present, to invoke the super class constructor. A single class can

    contain more then one constructor. That means constructor can be overloaded.

    There is no concept of destructor in JAVA. The concept of garbage collection deletes all the

    variables and functions of the program thats execution is completed. So all the instance

    variables are automatically initialized with there zero values at the time of creation of the

    object, because the memory is cleared by the garbage collection concept. If user wants to

    delete an object from the memory then thefinalize( ) function can be used to perform thosetype of operation. That means, using finalize () function, particular object can be deleted as

    the choice of the user but by the garbage collection all the objects are deleted after

    execution of the program which is performed automatically by theJVM.

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    Core Java Programming language Theory part only

    Generic programming is those type of programming concept by which we can create type

    independent code for all the type which maintains same algorithms. In JAVA programming

    language large number of generic classes is present to implement any type of concept.

    Generally the generic classes are vector, stack, queue, array list, circular queue, B tree etc.

    In JAVA the concept of generic functions is not present, only the concept of generic classes

    is present. Using this class user can handle any type of element as the requirement of the

    user.

    For example :- array is a generic class to handle any types of values using the concept of array.The array class contains a large number of methods to handle array elements. Thefill

    function is used to store values into the array. The sorfunction is used to arrange data

    elements in ascending or descending order. The delete function is used to erase data

    elements from the array. Similarly the search function is used to test existence of a

    particular element.

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    Core Java Programming language Theory part only

    Exceptions are a type of error besides then the syntax error or logical error as divide by zero.Alternately we can say that those types of error which are not belonging to syntax error or

    logical error are calledEXCEPTION. Exceptions are divide in to two categories :-

    1. Synchronous or software related exception.

    2. Asynchronous or hardware related exception.

    A synchronous type of exception is also called software exception. Which are arise due to

    software limitation, like arithmetic exception, input/output exception, number format

    exception etc. synchronous exceptions can be smoothly handled by JAVA programming

    language, because huge number system defined exception classes are present. The popularexceptions classes are input/output exception, arithmetic exception, number format

    exception, array index out of bound exceptions, string index out of bounds exception, class

    not found exception, sql exception etc. using system defined exception classes any type of

    exception can be handled besides then the system defined exception. User can also create

    his own exception classes to handle exceptions. In JAVA programming the following tools

    are used to handle exceptions.

    1. Try ( )

    2. Catch ( )

    3. Throw ( ) /Throws ( )

    The try block is used to identify exception. After identifying the exception JAVA automatically

    throws the exception to appropriate catch block, if the exceptions are system defined

    exception. If the exceptions are user defined exceptions then the throw statement is used to

    throw the exception to appropriate catch block. A single function can contain more then one

    try block and correspondingly more then one catch block. It is also possible to present more

    then one catch block corresponding to a single try block.

    Asynchronous exceptions are those types of exceptions which are arises due to hardware

    failure, which can not be handled by the JAVA programming language. Asynchronousexceptions can not be handled by any programming language.

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    Core Java Programming language Theory part only

    We know that methods are common for a set of object and instance variables are distinct for

    every object. Methods are allocated their memory at the time of class declaration and

    instance variables are allocated their memory at the time of object creation. In JAVAprogramming language objects are allocated their memory at the time of execution using

    the new operator. After allocation of memory the instance variables are binds with their

    associated methods at the time of execution. This type of data binding is also calleddynamic

    bindingorlate bindingorexecution time bindingorrun time binding. In JAVA object can

    not allocate their memory statically at the time of compilation. They allocate their memory

    at the time of execution. So only dynamic binding is present in JAVA. The concept of static

    binding is partially implemented using the concept of static variables and static function.

    Which are allocate their memory at the time of compilation and can be executed without

    object. The concept of dynamic binding is present in JAVA due to JAVA supports distributed

    processing. In distributed processing a large program can be decomposed to its smallersegments and distributed among all the processors which are connected with the server or

    main computer.

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    Core Java Programming language Theory part only

    Distributed processing is that type of processing in which more then one processor are involvedto execute a single program segment wise concurrently. In distributed processing a large

    program is divided to its independent smaller segment and processed by the processor,

    which are connected with the main server. All the segments are executed concurrently and

    at the end all the segments are combined and produce the actual output on the main server.

    The processing speed is rapidly increased due to the concept of distributed processing. To

    implement the concept of distributed processing the system should be in network. In JAVA

    programming language the distributed processing is implemented. Using the concept of

    applet andRMI(Remote Method Invocation) and using the concept of socket

    programming.

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    Core Java Programming language Theory part only

    We know that JAVA programming language is the combined form ofCandC++. The syntax cal

    concept of JAVA is arising from the C language. We know that the syntax of C programming

    language is very simple. This simplicity of syntax is arising in the JAVA programming

    language from the C language. The conceptual framework of JAVA is arising from C++,

    which are very simple concept. So the JAVA programming language is syntaxically and

    conceptually very simple.

    JAVA programming language provided high security to its data members and data values. We

    know that JAVA is a pure object oriented programming language. So, all the components

    must be present within class. We also know that classes are always encapsulated, so objects

    are always encapsulated. A cover is present around the components of objects, so any

    component can not be directly accessed.

    The visibility layer of JAVA programming like private, friendly, protected etc provides securityof the components. Inheritance and the concept of the package also increase the security of

    instance variables and methods. The applet programming can be executed on any web

    browser and can not perform any operation on the terminal on which it is executed. So,

    using the concept of applet information can be propagated in the entire network with

    affecting the network.

    User can developed any type of application using JAVA programming language. So we can say

    that JAVA is the most powerful programming language. JAVA language also has the

    capability to access native programming languages C, C++ etc as the choice of the user.

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    Core Java Programming language Theory part only

    The most important component of JAVA programming language isJVM (Java Virtual

    Machine).JVM is divided in three components-

    1. Java Compiler

    2. Java Interpreter

    3. Java Runtime Environment

    The JAVA compiler is responsible for to compile a source program to its equivalent

    intermediate code byte code, which is written in 16 bitcoding system Unicode. The byte

    code is understandable by any computer after interpreting to equivalent object code. The

    JAVA compiler compiles a JAVA program using two phase. The first phase is the analysis of

    source program in which syntax errors are checked. The next phase is the synthesis of objectprogram in which actual byte code equivalent is generated. JAVA compiler not compiles the

    system defined function, because they are in compiled form. The compiler is not compiled

    the whole program at a time, if the program is very large and divided by modules. Module

    by module compilation is performed by the JAVA compiler. It is also calledjust in time

    (JIT) Compiler.

    The next phase of execution of JAVA program is interpretation of byte code performed by JAVA

    interpreter(JAVA). The JAVA interpreter interprets the byte code to its equivalent object

    code understandable by the physical operating system and transfer to the JAVA run timeenvironment. That means JAVA program is executed in two phases-

    1. Compilation

    2. Interpretation

    So JAVA is compiled and interpreted.

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    The full form of applet is application layout. Applet is stand alone program which can be

    executed independently in any computer by using any web-browser like hot java, internet

    explorer, MSN messenger etc. applet program is stand alone program in which main

    function is not present and can be propagated in the internet automatically. System defined

    applet, class applet and j applet are present in the package appletjava andjavax. Applet

    program can not be created without the system defined class appletandj applet. The classes

    must be inherit in the user class to create applet program. The applet class generally

    contains the following popular functions:-

    1. start

    2. init3. paint

    4. repaint

    5. destroy

    The start( ) function start the execution of an applet program which is automatically called. The

    init( ) function initialize the applet to set the frame and visible the frame in the applet viewer

    and applet window. Thepaint ( ) function actually perform operation on the applet window

    as well as applet frame. The repaint( ) function is used to called the paint function again.

    The destroy( ) function kills the applet and stop viewing.Applet is divided in two categories according to the storage or location of the applet:-

    1. Local applet

    2. Remote applet

    Local applets are those types of applets which are stored in the same computer on which they

    are executing. If the system is stand alone, system can not connected in the network. User

    can create only local applet on that computer. Remote applets are those types of applets

    which are stored in the server and executing in any computer connected with the server.Generally websites are contains remote applet.

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    JAVA programming language is basically used to design the interface. JAVA is generally used

    as a front end program (software) in application development. JAVA programminglanguage has the capability to interact with anyDBMSorRDBMS. To store data values

    using the security layers of the DBMS. JAVA programming language has no such powerful

    capability to store data values in the files with maximum security. JAVA data file can be

    opened and modified by any user very easily but we know that in DBMS as well as RDBMS

    provides high security to the database using user name, password, privilege and rolls. So

    JAVA programmers are generally stored data values in data bases in theRDBMS, using the

    JAVA application program interface (A.P.I) JDBC (Java database connection).using

    JDBCuser can access andRDBMSandDBMSsuch as oracle, SQL server, MS access,

    Visual FoxPro etc as the choice of the user by selecting appropriate driver and set a

    connection with the databases. That means, JAVA API (Application Program interface)

    programs are present to interact with other programming languages as the choice of the

    user.

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    One of the most important features of any programming language is to implement the concept

    of object re-use ability. In JAVA programming language has the capability to re-use existingcode in other programs or in same programs. If an existing code is to be re-used in other

    program then the concept of package is arise. Package is the collection of related classes

    which is used to re-use existing code in other programs. JAVA programming language has

    huge number system defined packages. Which are contains system defined functions in

    appropriate classes. In JAVA programming language, near about162 System defined

    package are present and near about18000 System defined functions are present.

    The most popular System defined packages are applet, lang, util, swing, net, javax, awt etc.

    User can create his own package as the choice of the user. Package generally contains

    public classes otherwise the class can not visible in other program. The package statementmust be the first statement in the any user program. After that any statement can be present.

    User must be create a subdirectory as same as the package name and the program must be

    in the subdirectory with the name of the public class name with.(Dot) java extension. The

    class file of the program must be saved under the subdirectory. The package can be import

    in the super directory only. So, program in which the package is re-used should be created

    in the super directory of the package directory. At a time only one public class can be stored

    in a package. If the class is not public then it can not be import in the user program. Non

    public classes are hidden from the user program and they can not be accessed in the user

    program.

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    In JAVA programming language the concept of multiple inheritance is not present because

    more then one sub class can not be inherit at a time in a single sub class but more then one

    interface can be inherit in to a single sub class. That means using interface the concept of

    multiple inheritance can be implemented.

    Interface is also is an user defined data type as same as class. In which only constant value and

    abstract functions can be present. In interface the default visibility is public. Interface can

    contains function but all the functions are abstract by default. Functions present in the

    interface must be redefined in the sub classes otherwise error will arise. Data member

    present in the interface are by default static final. That means, only constant value can bepresent in interface. We can not change the data members in the sub classes of the interface.

    An interface can be inherit in to another interface using the keywordextends but more then

    one interface can not be inherit to a single interface at a time. An interface can be inherit in

    to a sub class using the keywordimplements.More then one interface can be inherit in to a

    sub class at a time. A system defined package event underawtpackage is present which

    contains all the system defined interfaces asActionListener, KeyListener, MouseListener,

    mousewheelListener, ItemListener, Componentlistener, WindowsListeneretc.

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